linux-kernel - Re: [PATCH 12/22] x86/fpu: Only write PKRU if it is different from current

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Message-ID: <198b2412-e81d-6c55-c74f-c2cab60fc5f2@intel.com>
Date:   Mon, 25 Feb 2019 10:08:10 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 12/22] x86/fpu: Only write PKRU if it is different from
 current

On 2/21/19 3:50 AM, Sebastian Andrzej Siewior wrote:
> @@ -111,6 +111,12 @@ static inline void __write_pkru(u32 pkru)
>  {
>  	u32 ecx = 0, edx = 0;
>  
> +	/*
> +	 * WRPKRU is relatively expensive compared to RDPKRU.
> +	 * Avoid WRPKRU when it would not change the value.
> +	 */
> +	if (pkru == __read_pkru())
> +		return;
>  	/*
>  	 * "wrpkru" instruction.  Loads contents in EAX to PKRU,
>  	 * requires that ecx = edx = 0.

Could we do this in write_pkru() instead?  I tried to keep the __
versions of read and write as simple and symmetric as possible.